GaN-based semiconductor materials are widely applied in semiconductor devices, such as LEDs, due to the property of wide band gap.
Conventionally, a GaN layer is formed on a sapphire substrate using metal organic chemical vapor deposition (MOCVD) techniques. Due to considerable difference in lattice parameters between the sapphire substrate and the GaN layer, lattice dislocation may occur in the GaN layer. In addition, due to the large difference in thermal expansion coefficient between the sapphire substrate and the GaN layer, stress is likely to occur at an interface between the sapphire substrate and the GaN layer, resulting in fracture of the GaN layer.
In order to overcome the aforesaid problems, a Ga2O3 substrate which has a lattice parameter similar to that of the GaN layer can be used as a substitute for the sapphire substrate.
However, the Ga2O3 substrate is likely to be degraded during epitaxial formation of the GaN layer under a H2 atmosphere and result in the formation of a discontinuous GaN layer.
To overcome the aforesaid degradation problem, a GaN seed layer with a thickness around 30 nm to 50 nm is first formed on the Ga2O3 substrate, followed by epitaxial growth of a GaN layer on the GaN seed layer. However, when the GaN seed layer is too thick, GaN island grains may be generated on a surface of the GaN seed layer and reduce the quality of the GaN layer. In contrast, when the GaN seed layer is too thin, the effect of preventing the Ga2O3 substrate from degradation may be reduced.
An alternative solution to the degradation problem is to form the GaN layer in an O2 atmosphere rather than in the H2 atmosphere. Unfortunately, the GaN layer thus formed has a cubic crystal structure that is worse in property than the hexagonal crystal structure formed in the H2 atmosphere.